There are two basic types of drill bits used for boring through subterranean rock formations when drilling oil and natural gas wells: drag bits and roller cone bits.
Drag bits have no moving parts. As a drag bit is rotated, typically by rotating a drill string to which it is attached, discrete cutting elements (“cutters”) affixed to the face of the bit drag across the bottom of the well, scraping or shearing the formation. Each cutter of a rotary drag bit is positioned and oriented on the face of the drag bit so that a portion of it, which will be referred to as its wear surface, engages the earth formation as the bit is being rotated. The cutters are spaced apart on an exterior cutting surface or face of the body of a drill bit in a fixed, predetermined pattern. The cutters are typically arrayed along each of several blades, which are raised ridges extending generally radially from the central axis of the bit, toward the periphery of the face, usually in a sweeping manner (as opposed to a straight line). The cutters along each blade present a predetermined cutting profile to the earth formation, shearing the formation as the bit rotates. Drilling fluid pumped down the drill string, into a central passageway formed in the center of the bit, and then out through ports formed in the face of the bit, both cools the cutters and helps to remove and carry cuttings from between the blades.
Roller cone bits are comprised of two or three cone-shaped cutters that rotate on an axis at approximately a thirty-five degree angle to the axis of rotation of the drill bit. As the bit is rotated, the cones roll across the bottom of the hole. Cutting elements—often referred to as teeth or inserts—on the surfaces of the cones crush and scrape the rock as they roll over the formation.
In order to improve performance of drill bits, one or more wear or working surfaces of the cutting elements are made from a layer of polycrystalline diamond (“PCD”) in the form of a polycrystalline diamond compact (“PDC”) that is attached to a substrate. A common substrate is cemented tungsten carbide. Drag bits with such PDC cutting elements are sometimes called “PDC bits.” PDC, though very hard with high abrasion or wear resistance, tends to be relatively brittle. The substrate, while not as hard, is tougher than the PDC, and thus has higher impact resistance. The length or height of the substrate is typically long enough to act as a mounting stud, with a portion of it fitting into a pocket or recess formed in the body of the drag bit or, the case of a roller cone bit, in the pocket formed in a cone. However, in some drag bits, the PDC and the substrate structure are attached to a metal mounting stud, which is then inserted into a pocket or other recess.
A polycrystalline diamond compact may be made by mixing polycrystalline diamond grains, in powder form, which is referred to as “diamond grit,” with one or more powdered metal catalysts and other materials, forming the mixture into a compact, and then sintering it using high heat and pressure or microwave heating. Although cobalt or an alloy of cobalt is the most common catalyst, other Group VIII metal, such as nickel, iron and alloys thereof can be used as catalyst. For a cutter, a PDC is typically formed by packing diamond grit, without the metal catalyst, adjacent to a substrate of cemented tungsten carbide, and then sintering the two together. During sintering metal binder in the substrate—cobalt in the case of cobalt cemented tungsten carbide—sweeps into and infiltrates the compact, acting as a catalyst to cause formation of diamond-to-diamond bonds between adjacent diamond grains. The result is a mass of bonded diamond crystals, which has been described as a continuous or integral matrix of diamond and even a “lattice,” having interstitial voids between the diamond. The interstitial voids are at least partly filled with the metal catalyst.
Substrates for supporting a PDC layer are made, at least in part, from cemented metal carbide, with tungsten carbide being the most common. Cemented metal carbide substrates are formed by sintering powdered metal carbide with a metal alloy binder. The composite of the PDC and the substrate can be fabricated in a number of different ways. It may also, for example, include transitional layers in which the metal carbide and diamond are mixed with other elements for improving bonding and reducing stress between the PDC and substrate. References herein to substrates include such substrates.
Because of the presence of metal catalyst, PDC exhibits thermal instability. Cobalt has a different coefficient of expansion to diamond. It expands at a greater rate, thus tending to weaken the diamond structure at higher temperatures. Furthermore, the melting point of cobalt is lower than diamond, which can lead to the cobalt causing diamond crystals within the PDC to begin to graphitize when temperatures reach or exceed the melting point, also weakening the PDC. To make the PDC more thermally stable, a substantial percentage—usually more than 50%; often 70% to 85%; and possibly more—of the catalyst is removed from at least a region next to one or more working surfaces that experience the highest temperatures due to friction during drilling. The working surfaces are the surfaces of the cutter designed or intended to engage the formation. In the case a PDC cutter, for example, they are typically the typically planar top surface of the diamond crown or table, at least part of its side surface, and, if present, a beveled edge, radiused or shaped transition between the top and side surfaces.
The catalyst is removed by a leaching process. The leaching process involves protecting, or masking, the carbide substrate and placing the PDC in a strong acid, examples of which include nitric acid, hydrofluoric acid, hydrochloric acid, or perchloric acid, and combinations of them. In some cases, the acid mix may be heated and/or agitated to accelerate the leaching process.
Removal of the cobalt is, however, thought to reduce toughness of the PDC, thus decreasing its impact resistance. Furthermore, leaching the PDC can result in removal of some of the cobalt that cements or binds the substrate, thus affecting the strength or integrity of the substrate and/or the interface of the substrate and diamond interface. As a result of these concerns, leaching of cutters is now “partial,” meaning that catalyst is removed only from the PDC to a certain depth or distance measured from a working surface or working surfaces of the PDC, such as the top, beveled edge, and/or side of the PDC. Generally, only the top, working surface of the PDC is exposed to the bath. A mask and seal are used to protect the substrate from the acid. The leach depths of conventional, commercially produced cutting elements are substantially consistent across the cutter as measured from the working surface exposed to the acid bath. The depth of the leaching depends on the microstructure of the diamond material, the leaching solution being used, and the leaching time.